High frequency attenuating device



Dec. 11, 1945. s. E. ARCHENBRONN HIGH FREQUENCY ATTENUATINGDEVICE 2 Sheets-Sheet 1 Filed June 21, 1945 INVENTOR. 620 9615. CZrc/zenbmm HMPLIFIER MULTI-VIBRHTOR Patented Dec. 11, 1945 2,390,489 HIGH FREQUENCY ATTENUATING nnvror.

George Edward Archenbronn, Chicago, Ill., us

signor, by mesne assignments, to Belmont Radio Corporation, Chicago,

nois

111., a corporation of Illi- Application June-21, 1943, Serial No. 491,614

1 Claim.

I'he present invention relates to variable high frequency attenuating devices and more particularly to an improved arrangement for variably attenuating the signal energy transfer between the output circuit of a highfrequency signal generator or the like and the pickup circuit of apparatus in which the transferred signal energy is to be utilized.

In certain applications it is necessary to utilize signal generators, which are capable of producing a radio frequency voltage which may be unmodulated, pulse modulated at a fixed but adjustable audio rate, or modulated with an audio voltage of fixed but adjustable audio frequency. These applications usually'requir'e that the signal energy delivered by the generator to the utilization circuit be variable within rather wide limits. To meet this requirement-the usual practice is that of providing a generator capable of producing a signal energy output exceeding that which is required; to operate the generator continuous-- ly at its maximum output; and to provide facilities for variably attenuating the energy transfer from the generator to the energy utilizing circuit. Various expedients have been resorted to in attempts to obtain the desired attenuation, all of which are somewhat complicated in character and are limited in the obtainable attenuation range.

It is an object of the present invention, there fore, to provide an improved and exceedingly simple arrangement for variably attenuating the energy transfer from a source of high frequency current to an associated pickup circuit.

In accordance with another object of the invention, an improved variable attenuating device is provided having an exceedingly wide range of attenuation.

According to a further and more specific object of the invention, the output circuit of a high frequency generator is provided with an output inductance which is coupled to the pickup inductance of the utilization circuit through an opening in one wall of the shielding housing for the high frequency section of the generator, and improved facilities are provided for variably closing the opening in order to vary the coupling between the two inductance elements and thus variably attenuate the energy transfer therebe-' tween.

In accordance with still another object of the invention, an improved arrangement is provided for minimizing stray coupling between the two inductance elements, thereby to render the obtained coupling directly dependent upon the size of the opening through which the two elements are intercoupled.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 illustrates in schematic form a signal generator and associated pickup facilities having provided therebetween a variable attenuation device characterized by the features of the pres ent invention;

Fig. 2 is a top perspective view illustrating the mechanical construction and arrangement of the apparatus schematically shown in Fig. 1;

Fi 3 is an end view of the attenuating device provided in the apparatus;

Figs. 4 and 5 are sectional views taken alon the line 4-4 of Fig. 3 illustrating in assembled and disassembled relationship, theelements of the attenuating device shown in Fig. 3;

Fig. 6 is a curve illustrating the operating characteristics of the attenuating device; and

Fig. 7 is a front plan view of the control panel forming a part of the assembly shown in Fig. 2.

Referring now to the drawings, and more par ticularly to Fig. 1 thereof, the invention is illustrated in its embodiment in a signal generator for the purpose of attenuating the signal energy transfer between the output side of the generator and an associated pickup circuit. In brief, the illustrated signal generator comprises a multivibrator or modulating stage I 0, an audio frequency amplifier II and a radio frequency carrier producing oscillator l2 connected in tandem in the order named. The audio frequency amplifier H and the multi-vibrator or modulating stage 10 are entirely conventional in character and, accordingly, have been only schematically illustrated. It is pointed out, however, that these two stages of the generator are arranged for plate circuit control of the oscillator l2 and include facilities for either pulse modulating the radio frequency output of the oscillator, or continuously modulating the generated radio frequency output of the oscillator at a fixed but adjustable audio frequency rate. To this end, suitable switching facilities are provided for rearranging the circuit elements of the two stages Ill and II so that either type of modulation of the signal ouput from the oscillator I2 is produced. Additional switching facilities are provided for changing the circuit constants of the'modulation frequency determining circuits provided in and grid of the tube I 3 through-a grid leak and condenser network l6 whichficomprisespthe leak, resistor l1 and shunt connectedfl condenser |8 For the purpose of variably. tuning the inductance:

element l4 so that the oscillator will oscillate at any desired radio frequency within at desired range, a variable tuning condenser 19 is provided which is connected between a inductance element and ground. The output circuit of=the audidfre'quency amplifier-H is bypassed for radio frequencyeurrents bycmeansof a. condenser 2L: From the above explanation; it will beaunderstood that the oscillator i2: is :in the nature of a conventional Hartley oscillator arranged for plate circuit-modulation; The: output :'voltage from this-oscillator is'deri'ved bytap? ping the inductance element 14 iatca point-2 2 therealong v and shunting the lowermost section I40 of this inductance element with an. output inductance element-23. As will be apparent from Figs: 1 and :2 of theldrawinga'all elements of the high frequency oscillator l2, includingthe-output inductance element 23;:aare providedwithin a conductive'housing -24 -which-is.maintained at the reference ground potential andaacts to shield the elementsgof. the osciliatorz against stray-coupling -with extraneous x circuits;iand cu rre nt so.urces;,-;:, 1 V

Fort-the ,purpose'ofutransferring renerg-ydrom theoutputinductanceelement 23'of,- the genera-tor to a utilization circuitxa pickup arrangement I is provided which includes-two. connectors 25c and 25b two shielded conductors'fia and 21a; anda pickupinductance-element 28-, the terminal: ends of which are connected respectively to the adiae cent ends ofthe conductors 25 a and --2 'a.; The two inductance elements. 23,- and -:28: which: may eachzcomprise only a single turnat the frequene cleszinvolved are disp sedi-n parallel-planes and in proximity to each: Other upon eitheiside 'of' the housing -wall Mat, They are mutually cou led through an opening Mbmrovided in this housing wall and having-"a center which is-substantially axial-ali nment with. the; centers-10f, the-(two elements, Primarily, the magnetic or mutu ailinductive coupling betweentheitwo elements 23 and 28 is: .relied .,upon= to obtain the desired energy transfer between the output. side. of the oscillator and. the-utilizat on circuit. in which-the pickup point 28 along the element 28ii=iprovided- Accordingly, it.is.n.eces- V sary to, provide facilities for suitably attenuating or varyingthiscouplin-gin order to adjust-the energy transfer :between the. two [inductance elernents. To this. end, an adjustable rotor plate, 29

s: provided for. variably clcsing the, opening 24b fully explainedbelow; g 7

As indicated above. the mechanical-construction andv arrangementof the'signal generator and its; associated pickup facilities-ere illustrated in upon "a metal chassis 30 which islprospaced-apart relation from the front housing wall 24a through which the two inductance elements 23 and 28 are coupled. All circuit elements of the oscillator I 2 are disposed within the conductive housing 24 and a suitable conductive lid, not shown, is provided for closing the top of the housing. in the completed generator; assembly. Thus, the osci1lator. tube l3 isamounted upon a socket which is supported upon the top wall of the chassis within the Walls of the housing 24; the frequency adjusting condenser I9 is supported upon the inner sideofthe front wall 24a of the housing; the tuning inductance element I4 is appropriately supported betweenthe rear wall of the housing, theucathodei terrninal of the tube socket and one terminal of the fr equency adjusting condenser I 9; and the ends of the output inductance element-23aresoldered to the inductance element l4 and the front housing wall 24a so that the single turn thereof is aligned with the opening 241); 'I'herremainingcircuitelements of the generatondncluding the elementsof-a suitable-power pack: and the elementsof the-two stages land It, aredisposed outside of [the housing grand certain thereof are disposed within the compartment' defined: by the base 30a of-the chassis-30;

As 'bBSt'IShO'WII in Fig. 7 *of the drawings the control elements fo the which is utilized to-fcondition the equipment for operation; to selectively arrange the equipment for" pulse or audio frequency modulation of the outputvoltage-and toa-vary the pulse or continu ous ,modulation frequency are :all-extended through the frontcon-trol panel 3! of the chassis for'easy manipulation by an operator. Thesecontrol-elements include the actuating armof anonoffswitch 3ft, the actuating arm ofa switch33 which iszutilized selectively toicondition the apparatus for pulse: ror'audio frequency modulation ofiithe; outputwoltage, andthe actuating knob ofza fiver-position rotary switch T34 which' is uth lized selectively to rearrange the-circuit elements of ,themul-tiwibraton I0 so that an unmodulated signalivoltageorn a modulated rvoltagea of any one of four different frequencies is 'produced by-the generator The fivemdi-fferentpositions of-the. switch 34 arerespe'ctively indicated by a scale 340. which is arranged to be indexed by a pointer carried by the actuating knob for the switch." The frontcontrol panel 3 I- of :the chassis 30 alsorhas an opening therein through which the rotor shaft of anadjustable tuning condenser: l 9 extends, this shaft being provided at its: extended end withan I Referring now more particularly :to the :me-

chanical, constructionand; arrangement of :the pickup facilities to which'signal energy-is transferredfrom the output inductance element 23 of the signalgenerator, it willgbe' noted that the two-connectors wand: ZShjareof the female t p and a s ta ly suppo ted upon the front control panel -3 l. assembly screwsor ,theslike -'I1 16SQ;1;W0 connectors; are respectively provide with tubular elements which are-disposed rearwardly of the panel 3!, and respectively terminate and support opposite ends of tubular metal shielding elements 26 and 21 within which i the conductors-16a and 21a are supported gby- .tmeans; :of. suitable insulating disks The inner-tends of these two tubular elepa iel SL-arrangeQ-in '7 merits 26, a d 21. ext ndlbetw emthe innensun switching apparatus 7 f th .cha-ss stciillby means of v face of the panel 3| and the outer surface of is provided with a slot in the wall thereof which faces the housing wall 24a, whereby the ends of the pickup coil 28 may be connected to the inner cable conductors 26a, and 21a.

Briefly to consider the mechanical construction and arrangement of the assembly for rotatably supporting the rotor plate 29 upon the front wall 24a of the conductive housing 24, it is pointed out that this assembly comprises a sleeve 31 having a hub of reduced diameter upon which the rotor plate 29 is rigidly mounted. This sleeve is rigidly mounted upon an operatin shaft 38 by means of a set screw 31a, and the shaft 38 is joumaled in a bearing 39 rigidly mounted upon the front wall 24a within the housing 24. The operating end of the shaft 38 extends through an opening 43 drilled through the control panel 3| and is adapted to receive an actuating knob 4! which is mounted upon the shaft end by means of a set screw 42. Suitably calibrated scales Ma and 4lb may be provided in indexing relationship with a pointer carried by the knob 4| in order to indicate the extent of attenuation obtained at any given setting of the rotor plate 29. For the purpose of retaining the enumerated parts of the rotor plate assembly in the desired assembled relationship, three assembly washers 43, 44 and 45 are provided, the first of which is of the split ring type and is adapted to be retained within an annular slot formed at the inner end of the shaft 38. The second washer 44 is strictly a friction washer and is adapted to engage the outer convex surface of the dish-shaped washer 45. The third washer 45 is adapted to be flattened during the assembly operation, for the purpose of spring biasing the control shaft 38 to the left as viewed in Figs. 4 and 5 of the drawings, and is utilized to maintain the rotor plate in engagement with the housing wall 24a, thereby to minimize stray coupling between the two inductance elements 23 and 29 in the manner more fully explained below.

As "best shown in Figs. 2 and 3 of the drawings, the rotor plate 29 extends within the lower compartment of the chassis 30 through a slot 46 cut in the top wall of the chassis, and overlies the opening 241) of the housing wall 24a through which the two inductance elements 23 and 28 are intercoupled. This plate is provided with an arcuate slot 41 which is of gradually decreasing width from the lower end to the upper end thereof, as viewed in Fig. 3 of the drawings, and is so formed that, regardless of 7 he position of the rotor plate, the portion thereof in registry with the opening 24b overlies the central portion of the opening. More specifically, the two edges 41a and 41b of the slot are eccentrically or non-concentrically disposed with respect to the center of rotation of the rotor plate, whereby portions of the rotor plate adjacent both edges of the slot overlie the opening 24b.

In order to prevent stray coupling between the two inductance elements 23 and 28, it is essential that the portions of the rotor plate which cover the opening 24!) be maintained in good physical and electrical contact with the adjacent portion of the housing wall 24a. To this end, the edge of the housing wall which defines the opening 24b is provided with a struck out flanged portion 240 which extends toward the adjacent surface of the rotor plate 29 and may be formed by a stamping operation or the equivalent. The inner surface of the rotor plate 29 is held in abutting contact with the outer surface of the flange 240 due to the spring bias exerted axially along the shaft 38 by means of the dished biasing washer 45. Thus the arrangement is such that all portions of the flanged edge 240 are held in tight engagement with the adjacent surface of the rotor plate 29, regardless of the position to which this rotor plate is adjusted through rotation of the knob and shaft assembly 4|, 38. In order to improve the electrical contact between the engaged surfaces of the flange 24c and the rotor plate 29, a heavy coating of silver or other precious metal contact material is provided on the outer annular surface of the flange 24c and at all points within the area of the rotor plate surface which may engage the plated surface of the flange 24c during rotation of this plate.

For the purpose of limiting the extent of rotation of the rotor plate 29, a suitable stop mechanism is provided which includes a stop pin 48 mounted upon the housing wall 24a and extending outwardly therefrom through an annular slot 49 cut in the rotor plate 29 concentrically about the center of rotation of this plate. The pin 48 is so positioned along the front housing wall 24a that when engaged by the end 49a of the slot 49, the opening 24b is substantially entirely closed by the rotor plate 29. At the other extreme position of the rotor plate 29, i. e. that position wherein the stop pin 48 engages the slot end 49b, the enlarged end portion 41d of the slot 41 overlies the opening 24b, but the edge 41c of this slot is removed a substantial distance from the adjacent edge of the opening 241). The stoppage of the slot edge t a point removed from the edge of the opening 241) is necessary in order to prevent the approaching end portion of the slot edge from producing rather a sharp change in the coupling between the two inductance elements with a resulting distortion of the attenuation curve characterizing the degree of energy transfer between the two elements.

In considering the operation of the apparatus described above, it will be understood that connections from the signal generator to a signal utilization circuit may be made by appropriately plugging male type of connectors into the female connectors 25a and 25b. Also, by suitable manipulation of the switch 33, the switch 34 and the condenser adjusting knob 35, the equipment may be conditioned to generate, at any desired carrier frequency, an unmodulated radio frequency carrier voltage, a carrier voltage which is pulse modulated at a desired rate, or a carrier voltage which is continuously modulated at a desired frequency rate. In this regard it will be understood that the selective actuation of the switch 33 to either of its two operating positions determines whether the apparatus will produce a pulse or continuously modulated carrier voltage; that the setting of the switch 34 in one of its five positions determines whether the generated carrier voltage shall be unmodulated or shall be modulated at a particular frequency; and that the setting of the condenser adjusting knob 35 determines the particular carrier frequency at which the signal voltage shall be generated. It will also be apparent from the preceding explanation that the setting of the rotor plate adjusting knob 4| will determine the extent of coupling between the output inductance element 23 and the pickup inductance element 28 and, hence, the

;tuning condenser energy j-transfer ibetween" the output sideof :the

generator :and the" utilization circuit:

After the above adjustments have been amade, the-switch 32' may be actuated to its on position for-thepurposeiof-deliveringcathode-heatingcur .rent 'to the tubes provided in the *three stages :se'ctionaMa of the inductance element M.- This section of the rinductanceelement isinductively coupled to the upper section 14b thereof so'that radio ,frequency {energy may :be" transferred-:fr'om the output circuit 'to .theinputcircuit et 'the tube-l3 :for the *purpose of' sustaining the generatediosci-llatory'voltage. The modulating volt- *age'developed through operation of the multivibrator 4!} is amplified through the audio "fre quencyiamplifier 1| l and is -util-ized for plate circuitmodulation of the carrier'frequency voltage developed between the anode and cathode of the oscillator tube-il 2; This'modulating Voltage may either be utilized to continuously modulate the voltageappearing across the inductance element section I'4a -ata frequency determined by the setting of the rotary switch 34, orto pulse modulate the radio -frequency yoltage. developed by the oscillator l2 at-a --particular frequency. determined by the :settingof the switch -34, The type -of modulation obtained depends upon the particular .positionoccupiedby theswitch 33.: It willhe understood, therefore, that, depending upon the setting of the switch 33, a-pulse or continuously modulated radio frequency carrier voltage is developed across the lower section Ma of the inductance element 14, i. e. that portion of the inductance element I4 which is includedin the output circuit of the tube l3. A portion of this modulated voltage, i. e. that appearing across the section I40 of the inductance element I4, is impressed directly across the terminals of the output inductance element 23'. Due to the mutual inductive coupling which exists between the two inductance'elements 23 and 28 through the opening 2%, a voltage isjdevelcped' betweenjthe terminal endsof the pickup inductance element 28. This'voltage is impressed upon the utilization circuit through the conductors 26a and 21a and the inner contacts of the connectors 25d and 25b to which these conductors are respectively connected; The magnitude of this voltage and,

hence, the extent of energy transfer between the two inductance elements 231. and '28is of course determined by the amount ofinductive coupling therebetween, and this coupling is in turn determined by the setting of the rotor plate 29. More specifically ,-if the rotor plate is's'o adjusted that the'narrow end portion 41c of the slot 41 overlies the opening 24!), whereby the effective coupling .area between-the two -coils- 23 and 28 is small, only a negligible amount of coupling is present between the two coils 23 and 28 and, accordingly, the voltage developed between the terminals of the element 28 is of a low order of magnitude. On the other hand, if the rotor plate 29 is sol adjusted thatthe'portion of the slot 41 of maximum" width overlies the opening 24b, a substantial coupling area between the two inductance elements 23 and 28 is provided,- T such that the "maximum voltage :is developedbetwee'n the -;ter'- mi'nals of therinductance "element '-28'.= As the rotor plate is adjusted topositions intermediate the two described extremepositio'n's, the coupling between the two inductance elements 23' and'2'8 is correspondingly varied between" the minimum and maximum values tocorrespondin'gly change the voltage produced across the 'terminals oflthe pickup inductance element 28.

While it will be 'understood'th'at th structural specifications may :vary according to the design of a particularapplication for the 'a'ttenuatih'g device, the followin specifications' for the illustrateddevice are given by way of example as be ing satisfactory when used in conjunction -with a signal generator having 'an output carrierire- 'quency variable between Land 250 megacycles which 'may- "be continuously orpulse modulated "at a fr'equency range extending from 100 cycles toa maximum value-of 100 kilocyclesr 7 Output inductance element 23: a

A single loop of Nopllcopper tinned wire having "an inside diameter of .4375 Pickup inductance element 28:

A single loop of No.14 copper tinned V wire having an inside diameter" f of a .4375? Diameter of the opening 24b .875" Distance between inductance elements merits 2s and 2a-" .3125" Mean radius of slot 67-; 1.25? Maximum width of slot '4i 1 525"" Minimum width of slot EL .0937" Angle covered by slot 4l r lAp. 19 5" Angle oi rotation-of plate 29;. -Ap.i

Referring now more particularly to Fig. '6 "of the drawings, the attenuation curve ther illus trate'd-is an exact reproductionof acurve ob tained' byxtesting aworking embodiment ofthe invention characterized bythe physical 'andstruc- .tural dimensions outlined above. Inthis curve, the 'signal' attenuation from the available m'a'xi mum value as measured in decibels is plotted "as a function of the position of the rotor plate '29.

From an examination'pf this curve it will be apparent that when the rotor plate occupies the position in which the opening 24b is substantially closed such that minimum couplin between the two inductance elements is' obtained, the attend ation of signalenergy transfer is at a maximum which is of the'order of 58 decibels; As the-rotor plate is rotatedto increase the area ing through which the two elements the attenuation of energy' transfer'is sharply re duced'. Asthe'rotor platejsrotat'e'd to a position in which the maximurn' 'coupling area is'provided,

the attenuation is substantiallynegligible. From an examination of this curve and 'a further'con siderati-on of the configurationoftheslot'47,it will be understood that by suitablyflsh'apingthis slot, any desired characteristic attenuation curve may be obtained. Thus, by a propriate design'of the slot configuration, there maybe obtained an attenuation curve which is substantially linear from the point of maximum attenuation to' the point of minimum attenuation.

From the foregoing explanation, it will be un-' derstood that the described "attenuating device is particularly applicable for use-in governing the signalenergy transfer'betweenithe output side of a signal generator or other source of 'highfre quency current and an associated pick-up circuit;

It will also be understoodthat the device "is; "of

of the open: are coupled,

rugged structure and may be constructed to provid an extremely wide range of attenuation. Moreover, it is thoroughly reliable in operation and may easily be calibrated to provide a visual indication of the extent of attenuation for any setting of the rotor plate 29 embodied therein.

thereof, a chassis supporting said housing and provided with a control panel spaced apart from said one wall of said housing, terminal connectors supported upon said panel, shielded conductor sections extending between said wal1 and said panel from said connectors to points in proximity to said inductance element, a pickup inductance element connected between the adjacent ends of said conductor sections and disposed between said one wall and said panel in proximity to said output inductance element, said one wall being provided with an opening therein through which said inductance elements ar mutually coupled, and means for variably closing aid opening, thereby to vary the coupling between said inductance elements.

GEORGE EDWARD ARCHENBRONN. 

